My problem is getting dimension entities and text to appear at their proper size when a steel part 78" long is scaled down to fit on an 8.5" x 11" page. The arrowheads and the numerals shrink along with the part, and vanish into a blur hardly wider than the pen weight. The extension lines merge with the part itself.

Below is an excerpt from the QCAD book about text sizes. It also applies to dimension sizes (dimension label text height, arrow size, etc).Dimension sizes are configurable per drawing under Edit > Drawing Preferences > Dimension > Dimension Sizes.

Text Height

Before you create any text labels, you have to decide what text height to use. In a CAD system, you draw all objects at their actual size. When you print a drawing, you scale it up or down to fit on a sheet of paper. Naturally, text labels are scaled up or down together with the rest of the drawing.

If you draw a floor plan of a house, your text labels probably have to be about 25cm high. When printing the drawing, QCAD scales it down for example by a factor of 100 (1:100). The 25cm text labels are printed 2.5mm high, that is 100 times smaller.

On the other hand, if you draw a small mechanical part at a scale of 10:1, your text labels should be about 0.25mm high to print as 2.5mm on paper.

Remember to always keep the scale factor of your drawing in mind when creating text labels, dimensions or hatch patterns.